1. Technical Field
The present invention is related to a digital speaker driving apparatus, a digital speaker apparatus, an actuator, a flat display apparatus, and a mobile electronic device. Specifically, the present invention is related to a digital acoustic system and applications thereof in which a digital speaker apparatus which directly converts a digital signal into an analog audio is used.
2. Description of the Related Art
The digital speaker technology which directly converts a digital signal into an analog audio has been proposed.
In WO2007/135928A, a method is shown in which an analog audio is directly converted with a circuit inputting two digital audio signals X (L) and Y (R) and outputting a plurality of digital signals, the circuit comprising a ΔΣ modulator and a miss-match shaping filter circuit, and a plurality of speakers driven by the plurality of digital signals or a plurality of driving elements.
In addition to having a characteristic of low power consumption compared to an analog speaker apparatus driven by an analog electric signal, because a plurality of speaker elements or plurality of driving elements (coils etc) are used, such a digital speaker apparatus, which directly converts a digital signal into an analog audio by using the digital speaker technology, can produce a large sound pressure at a low driving voltage, compared to a speaker which uses a conventional one speaker element or single driving element driven by an analog electric signal.
However, because it is presumed that an analog signal is input in a conventional speaker apparatus, it is necessary to convert the input analog signal into a digital signal in order to integrate a digital speaker apparatus into a conventional digital acoustic system, a significant problem arises when realizing a digital acoustic system which uses a digital speaker apparatus.
Typical examples of digital acoustic systems which use conventional analog speaker apparatuses are shown in FIG. 1a to FIG. 1c. FIG. 1a shows a first embodiment of a digital acoustic system which uses a digital signal source and an analog speaker apparatus. In this example, a signal from the digital signal source is converted to an analog signal by a D/A convertor and the signal is input to an analog speaker apparatus after the amplitude of the signal is amplified by an analog amplifier. This embodiment is best suited to an acoustic system which uses a conventional analog amplifier and a speaker.
FIG. 1b shows a second embodiment of a digital acoustic system which uses a digital signal source and an analog speaker apparatus. In this example, a digital signal from a digital signal source is converted to an analog signal by a D/A convertor and input to an amplifier. In the amplifier, after the input analog signal is once converted to a digital signal by an A/D convertor, the signal is PWM modulated and the amplitude digitally amplified (D-class amplification) and then converted back to an analog signal by the D/A convertor (usually an LC filter) and input to an analog speaker apparatus. It is a more complex system compared with FIG. 1a, however, the digital amplitude amplification (D-class amplification) has higher power efficiency and the amplifier can operate with less power and with smaller space compared to an analog amplifier and therefore has become gradually used in recent years.
FIG. 1c shows a third embodiment of a digital acoustic system which uses a digital signal source and an analog speaker apparatus. In this example, a signal from the digital single source is input directly as a digital signal to an amplifier. In the amplifier, after the input signal is PWM modulated, the signal is converted to an analog signal by a D/A convertor (usually an LC filter) after the amplitude of the signal is digitally amplified (D-class amplification) and input to an analog speaker apparatus. In a digital device such as a personal computer or a mobile phone, a digital signal source and an amplifier for the internet or data transmission are often arranged within the same device and this type of system is being widely used.
In FIG. 1b and FIG. 1c, after the amplitude is digitally amplified (D-class amplification), a signal is converted to an analog signal by a D/A convertor (usually an LC filter) and input to an analog speaker apparatus, however, the digital amplitude amplification must be done with a high voltage in order to output a large sound pressure, a digital amplitude amplification semiconductor device which uses a high voltage special semiconductor technology is required and it is difficult to lower costs. In addition, when digital amplitude amplification is performed with a high voltage, electromagnetic irradiation becomes large and additional costs are incurred in order to reduce this electromagnetic irradiation.
In these conventional examples, because an analog signal is used as a signal input to a speaker apparatus, it is necessary to convert the analog signal which is input to the speaker apparatus to a digital signal when a digital speaker apparatus which uses a circuit which outputs a plurality of digital signals by a ΔΣ modulator and a miss-match shaping filter circuit as disclosed in WO2007/13528A1, and a plurality of speakers driven by a plurality of digital signals is used, and therefore an A/D convertor must be further added to realize the digital speaker apparatus and the power consumed by the newly required A/D convertor increases and the digital speaker apparatus becomes complex. In addition, there is a problem in which a deterioration in sound quality between an analog signal and a digital signal in the A/D (D/A) convertor between the digital signal source and the digital speaker apparatus can not be avoided.